Continuous casting of non-deoxidized steel and weakly deoxidized steel, such as rimmed steel and semi-rimmed steel, has a drawback reflected in the operation and quality of the steel. This drawback, which involves the generation of bubbles during the casting of the steel, or the fact that bubbles remain in the cast steel to cause trouble, has prevented the continuous casting of non-deoxidized steel and weakly deoxidized steel from being practically carried out. However, various investigations have recently been made with respect to the technique for removing gas in molten steel by circularly moving (stirring) the molten steel in a continuous casting mold by means of an electromagnetic stirrer (a large number of the investigations have been actually reported). Similar to the above described method and apparatus for stirring electromagnetically molten steel in a casting mold, there are other various methods and apparatuses. However, when the effect for improving the operability and the quality of cast steel was taken into consideration, a circular flow which rotates on a horizontal plane as illustrated in FIG. 1 was most effective. In the stirring technique illustrated in FIG. 1, electromagnetic stirrers 3 and 3' are oppositely arranged on the walls of both long sides 2a and 2a' of a casting mold 2, and electromagnetic forces 4 and 4' which are in reverse directions with each other act on the molten steel flow, whereby the molten steel 1 is moved in a direction indicated by the arrows 5 and 5' and is stirred. When such flow is caused in the molten steel, bubbles caught in the vicinity of the solidifying interface are again washed and moved and are promoted to be floated up to the molten steel surface so that the bubbles contained in the molten steel are effectively removed. The flow rate of the molten steel necessary for removing bubbles is about 0.2-1.0 m/sec, and is generally preferred to be at least 0.5 m/sec.
FIGS. 2 and 3 illustrate the distribution of flow rate of molten steel in the flow illustrated in FIG. 1. FIGS. 2 and 3 illustrate the distribution of flow rate at the initial stage of acceleration when the average flow rate of the molten steel is 0.5 m/sec. Furthermore, the flow rate distribution is not constant even in the thickness direction (y direction in FIG. 1) of the cast steel, but has a distribution illustrated in FIG. 3. Accordingly, when the flow rate distribution in the width direction (x direction in FIG. 1) of cast steel is shown by using positions (a and b in FIG. 3), at which the flow rate becomes maximum (v.sub.max) and the average flow rate becomes minimum (v.sub.mean) (as representative points), the flow rate distribution shown in FIG. 2 is obtained.
It can be seen from FIG. 2 that, in such prior techniques, the flow rate is short in the first half (E-L) of acceleration, and is excessively high in the second half (L-F) thereof. Particularly, the flow rate becomes a maximum flow rate of 1.4 m/sec and is about 3 times the amount of the average flow rate at the position at which the molten steel collides with the short side wall (2b) in the finishing stage (F-B) of acceleration. When the rate of the circulating flow of molten steel in a casting mold along the wall in the horizontal direction is not uniform, the following troubles occur. That is, at a short flow rate position, bubbles can not be fully removed, and surface defects, such as pin holes and the like, are caused; and reversely, at an excessively high flow rate position, troubles, such as slag patches, formation of oscillation marks and the like, are caused due to the lap of powder and the like. Particularly, at the collision portion of molten steel flow with the short side wall 2b, lap of powder is apt to be caused due to the jumping of molten steel.
In order to resolve the above described various problems, there has been proposed a method, wherein an electromagnetic stirrer 3 is rotated at a constant stirring strength in order to minimize the adverse effects due to the non-uniform flow rate of molten steel in the width direction of the cast steel.
However, although maintaining the stirring strength at a constant value can control the stirring rate, unevenness of flow rate due to the difference of positions can not be overcome. Therefore, the above described problems could not have been fundamentally solved by the aforementioned method.
In order to prevent the jumping of molten steel surface at the collision portion of the molten steel flow with the short side wall, there has been proposed a technique, wherein the short side walls 2b and 2b' are made into a semi-circular shape, or are cut down at the corner portions as illustrated in FIGS. 5 and 6, whereby the circulating flow of molten steel is made smooth to prevent the jumping of the molten steel surface.
However, in many molds for casting slab, the short side wall 2b is formed of a separated part as illustrated in FIG. 6 so that the width of cast steel can be changed. Accordingly, if the short side wall 2b is made into a semi-circular shape, both end portions of the short side wall (the portion shown by A in FIG. 6) have a very small thickness and are easily melted and broken, or deformed. Moreover, it is practically difficult to produce a short side wall having such shape. In order to obviate this problem, a casting mold having a shape illustrated in FIG. 5 is generally and practically used. In this case, the jumping of the molten steel surface at the collision portion of the molten steel flow with the short side wall can not be fully prevented, and the use of a casting mold having such structure alone can not fundamentally solve the problem.
The present invention intends to obviate the above described drawbacks of the conventional technique for stirring molten steel in a casting mold, and provides an electromagnetic stirring method for molten steel and an apparatus used for the method. According to the present invention the flow of molten steel in the width direction of cast steel (long side wall side of a mold) is made as uniform as possible to prevent the above described drawbacks of the cast steel (due to the non-uniform flow rate in the conventional method), and at the same time the flow rate of molten steel at the collision portion with the short side wall is decreased to prevent the formation of surface defects of cast steel due to jumping of the molten steel surface.